a) Field of the Invention
The invention is directed to a radiation source for the generation of short-wavelength radiation in which a radiation-emitting plasma is enclosed by a vacuum chamber that is outfitted with at least one feed line and one outlet line for a buffer gas in order to ensure protection against debris for at least one optical element which directs the radiation to a radiation outlet opening in the vacuum chamber.
b) Description of the Related Art
Apart from a desired short-wavelength radiation, plasma-based radiation sources, in particular plasmas which are excited by laser radiation, also emit high-energy particles (debris) which can lead to damage and to curtailment of the life of the optical elements, particularly collector optics, that are provided for utilization of the radiation.
When using a buffer gas reduce debris, depending on the density of the buffer gas and on the length of the optical path, a significant portion of the desired radiation is absorbed along the path that must be traversed by the radiation emitted by the plasma through the buffer gas before reaching the intended area via the collector optics which are constructed as reflection optics. As a result, the efficiency of the radiation source is reduced.
In addition, the conflict between improved protection and increased absorption loss arising when buffer gas density is increased is aggravated by the demand for providing a very large divergence angle of the beam bundle in one direction from the plasma to the collector optics but only a small divergence angle in the opposite direction (from the plasma to the intermediate focus of the collector optics).
A compromise with respect to the buffer gas density which takes into account the requirement for adequate protection while keeping absorption losses within reason is not an acceptable solution because the efficiency of the radiation source is the primary aim of development.
Another unsatisfactory solution is presented in EP 1 329 772 A2 in which the priority is to prevent a mixing of the buffer gas with the work gas and in which the plasma source and optical element are separated from one another spatially by an area through which a flow of buffer gas, e.g., argon, is directed perpendicular to the radiating direction.